Identification of higher order long-propagation-length surface plasmon polariton modes in chemically prepared gold nanowires

Perspective on Tailored Nanostructure-Dominated SPP Effects for SERS

Localized surface plasmon resonance (LSPR) excited by an incident light can normally produce strong surface-enhanced Raman scattering (SERS) at the nanogaps among plasmonic nano-objects (so-called hot spots), which is extensively explored. In contrast, surface plasmon polaritons (SPPs) that can be generated by an incident beam via particular structures with a conservation of wave vectors can excite SERS effects as well. SPPs actually play an indispensable role in high-performance SERS devices but receive much less attention. In this perspective, SPPs and their couplings with LSPR for SERS excitations with differing effectiveness through particular plasmonic/dielectric structures/configurations, along with relevant fabrication approaches, are profoundly reviewed and commented on from a unique perspective from in situ to ex situ excitations of SERS enabled by spatiotemporally separated multiple processes of SPPs. Quantitative design of particular configurations/architectures enabling highly efficient and effective multiple processes of SPPs is particularly emphasized as one giant leap toward ultimate full quantitative design of intrinsically high-performance SERS chips and very critical for their batch manufacturability and applications as well. The viewpoints and prospects about innovative SERS devices based on tailored structure-dominated SPPs effects and their coupling with LSPR are presented and discussed.

Surface plasmon polaritons of higher-order mode and standing waves in metallic nanowires

The surface plasmon polaritons (SPPs) of higher-order mode propagating along a plasmonic nanowire (NW) or an elongated nanorod (NR) are studied theoretically. The dispersion relations of SPPs in NWs of different radii, obtained from a transcendental equation, show that the propagation lengths of SPPs of mode 1 and 2 at a specific frequency are longer than that of mode 0. For the higher-order mode, the spatial phase of the longitudinal component of electric field at a cross section of a NW exhibits the topological singularity, which indicates the optical vortex. Of importance, the streamlines of Poynting vector of these SPPs exhibit a helical winding along NW, and the azimuthal component of orbital momentum density exists in the nearfield of NW to produce a longitudinal orbital angular momentum (OAM). Two types of standing wave of counter-propagating SPPs of mode 1 and 2 are also studied; they perform as a string of beads or twisted donut depending on whether the handedness of two opposite-direction propagating SPPs is same or opposite. In addition, a SPP of mode 1 propagating along an elongated NR can be generated by means of an end-fire excitation of crossed electric bi-dipole with 90° phase difference. If the criterion of a resonator for a mode-1 standing wave (string of beads) is met, the configuration of a plasmonic NR associated with a pair of bi-dipoles with a phase delay (0° or 180°) at the two ends can be applied as a high-efficiency nanoantenna of transmission. Our results may pave a way to the further study of SPPs of higher-order mode carrying OAM along plasmonic waveguides.

DNA-Guided Room-Temperature Synthesis of Single-Crystalline Gold Nanostructures on Graphdiyne Substrates

Nobel metal nanoparticles with tunable morphologies are highly desirable due to their unique electronic, magnetic, optical, and/or catalytic features. Here we report the use of multilayered graphdyine (GD) as a substrate for the reductant-free, room-temperature synthesis of single-crystal Au nanostructures with tunable morphology. We find that the GD template rich in sp-carbon atoms possesses high affinity with Au atoms on the {111} facets, and that the intrinsic reductivity of GD facilitates the rapid growth of Au nanoplates. The introduction of single-stranded DNA strands further results in the synthesis of Au nanostructures with decreased anisotropy, i.e., polygons and flower-like nanoparticles. The DNA-guided tunable Au growth arises from the strong adsorption of DNA on the GD template that alters the uniformity of the interface, which provides a direct route to synthesize Au nanostructures with tailorable morphology and photonic properties.

Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures

This manuscript describes a reversible wet chemical process for the tip-selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip-selective dissolution was achieved by oxidation of AuNRs with a Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on the AuNR surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. A decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in a blue shift in the longitudinal plasmon band (LPB) position, and red shifts in the LPB position were achieved by increasing the AR by 1D growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when the desired AR and/or LPB position is achieved. In addition, the tip-selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long AuNWs.

Silver nano-needles: focused optical field induced solution synthesis and application in remote-excitation nanofocusing SERS

Tapered metallic nanostructures that harbor surface plasmons are highly interesting for nanophotonic applications because of their waveguiding and field-focusing properties. Here, we developed a focused optical field induced solution synthesis for unique crystallized silver nano-needles. Under the focused laser spot, inhomogeneous Ag monomer concentration is created, which triggers the uniaxial growth of silver nanostructures along the radial direction with decreasing rate, forming nano-needle structures. These nano-needles are several micrometers long, with diameter attenuating from hundreds to tens of nanometers, and terminated by a sharp apex only a few nanometers in diameter. Moreover, nano-needles with atomically smooth surfaces show excellent performance for plasmonic waveguiding and unique near-field compression abilities. This nano-needle structure can be used for effective remote-excitation detection/sensing. We also demonstrate the assembling and picking up of nano-needles, which indicate potential applications in intracellular endoscopy, high resolution scanning tips, on-chip nanophotonic devices, etc.

Ultrafast measurements of the dynamics of single nanostructures: a review

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.

Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications such as plasmon mode excitation, hot carrier generation, and construction of ultracompact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate-Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.

Optics of exciton-plasmon nanomaterials

This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued.

Plasmon transmission through excitonic subwavelength gaps

We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction.

Coupling Emitters and Silver Nanowires to Achieve Long-Range Plasmon-Mediated Fluorescence Energy Transfer

The development of quantum plasmonic circuitry requires efficient coupling between quantum emitters and plasmonic waveguides. A major experimental challenge is to simultaneously maximize the surface plasmon propagation length, the coupling efficiency into the plasmonic mode, and the Purcell factor. Addressing this challenge is also the key to enabling long-range energy transfer between quantum nanoemitters. Here, we use a dual-beam scanning confocal microscope to carefully investigate the interactions between fluorescent nanoparticles and surface plasmons on single-crystalline silver nanowires. By exciting the fluorescent nanoparticles via nanowire surface plasmons, we maximize the light-matter interactions and reach coupling efficiencies up to 44% together with 24× lifetime reduction and 4.1 μm propagation lengths. This improved optical performance enables the demonstration of long-range plasmon-mediated fluorescence energy transfer between two nanoparticles separated by micrometer distance. Our results provide guidelines toward practical realizations of efficient long-range fluorescence energy transfer for integrated plasmonics and quantum nano-optics.

Imaging nano-objects by linear and nonlinear optical absorption microscopies

Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.

Coupling of individual quantum emitters to channel plasmons

Efficient light-matter interaction lies at the heart of many emerging technologies that seek on-chip integration of solid-state photonic systems. Plasmonic waveguides, which guide the radiation in the form of strongly confined surface plasmon-polariton modes, represent a promising solution to manipulate single photons in coplanar architectures with unprecedented small footprints. Here we demonstrate coupling of the emission from a single quantum emitter to the channel plasmon polaritons supported by a V-groove plasmonic waveguide. Extensive theoretical simulations enable us to determine the position and orientation of the quantum emitter for optimum coupling. Concomitantly with these predictions, we demonstrate experimentally that 42% of a single nitrogen-vacancy centre emission efficiently couples into the supported modes of the V-groove. This work paves the way towards practical realization of efficient and long distance transfer of energy for integrated solid-state quantum systems.

Gold Nanobipyramid-Directed Growth of Length-Variable Silver Nanorods with Multipolar Plasmon Resonances

We report on a method for the preparation of uniform and length-variable Ag nanorods through anisotropic Ag overgrowth on high-purity Au nanobipyramids. The rod diameters can be roughly tailored from ∼20 nm to ∼50 nm by judicious selection of differently sized Au nanobipyramids. The rod lengths can be tuned from ∼150 nm to ∼550 nm by varying the Ag precursor amount during the overgrowth process and/or by anisotropic shortening through mild oxidation. The controllable aspect ratios, high purity, and high dimensional uniformity of these Ag nanorods enable the observation of Fabry-Pérot-like multipolar plasmon resonance modes in the colloidal suspensions at the ensemble level, which has so far been demonstrated only on Au nanorods prepared electrochemically with anodic aluminum oxide templates. Depending on the mode order and geometry of the Ag nanorods, the multipolar plasmon wavelengths can be readily tailored over a wide spectral range from the visible to near-infrared region. We have further elucidated the relationships between the multipolar plasmon wavelengths and the geometric dimensions of the Ag nanorods at both the ensemble and single-particle levels. Our results indicate that the Au nanobipyramid-directed, dimensionally controllable Ag nanorods will be an attractive and promising candidate for developing multipolar plasmon-based devices and applications.

Silver nanowires as receiving-radiating nanoantennas in plasmon-enhanced up-conversion processes

We demonstrate efficient coupling between plasmons in a single silver nanowire and nanocrystals doped with rare earth ions, α-NaYF4:Er(3+)/Yb(3+). Plasmonic interaction results in a sevenfold increase of the up-converted emission of nanocrystals located in the vicinity of the nanowires as well as much faster luminescence decays. The enhancement of the emission can be precisely controlled by the polarization of the excitation laser and is significantly stronger for polarization parallel to the nanowire antennas. Imaging of angular-resolved emission patterns in the Fourier plane reveals plasmon-mediated luminescence, where the up-converted radiation is emitted via the nanowire antennas as leakage radiation.

Design of oligoaziridine-PEG coatings for efficient nanogold cellular biotagging

Biocompatible oligoaziridine-PEG coated gold nanoparticles overcome self-quenching while targeting the cell nucleus. The course of gold biotags within the cell's environment was tracked through confocal laser microscopy.

Independently analyzing different surface plasmon polariton modes on silver nanowire

In this paper, surface plasmon polariton (SPP) modes on silver nanowire (AgNW), with different field symmetric, are studied by different near field methods, respectively. In the experiment, the excitation and detection of SPPs are performed by two probes of near field scanning optical microscope (NSOM) simultaneously, which realizes the study of SPPs in complete near field. By controlling the experimental conditions, two of the fundamental SPP modes are detected separately and their intensity distributions on AgNW are given by the NSOM images. In the discussion, creeping wave (CW) is introduced to analyze the experimental results, which improves the coincidence of the experimental results and the theoretical calculations. A detailed characterization of SPPs modes in near field, which gives a further insight into optical properties of AgNW, will benefit integrated optical circuits.

Tunable plasmon modes in single silver nanowire optical antennas characterized by far-field microscope polarization spectroscopy

Performing far-field microscope polarization spectroscopy and finite element method simulations, we investigated experimentally and theoretically the surface plasmon modes in single Ag nanowire antennas. Our results show that the surface plasmon resonances in the single Ag nanowire antenna can be tuned from the dipole plasmon mode to a higher order plasmon mode, which would result in the emission with different intensities and polarization states, for the semiconductor quantum dots coupled to the nanowire antenna. The fluorescence polarization is changed with different polarized excitation of the 800 nm light beam, while it remains parallel to the Ag nanowire axis at the 400 nm excitation. The 800 nm incident light interacts nonresonantly with the dipole plasmon mode with the polarized excitation parallel to the Ag nanowire axis, while it excites a higher order plasmon mode with the perpendicular excitation. Under excitation of 400 nm, either the parallel or perpendicular excitation can only result in a dipole plasmon mode. In addition, we demonstrate that the single Ag nanowire antenna can work as an energy concentrator for enhancing the two-photon excited fluorescence of semiconductor quantum dots.

Second-order surface-plasmon assisted responsivity enhancement in germanium nano-photodetectors with bull's eye antennas

The enhancement of photo-response in nanometer-scale germanium photodetectors through bull's eye antennas capable of supporting 2nd-order Bloch surface plasmon modes is demonstrated in theory and experiment. A detailed numerical investigation reveals that the presence of surface wave and its constructive interference with the directly incident light are incorporated into the main mechanisms for enhancing transmission through the central nanoaperture. With a grating period of 1500 nm, the area-normalized responsivity can be enhanced up to 3.8 times at 2 V bias for a 780 nm laser. It provides an easier fabrication path for ultra-short wavelength operations especially in devices using optically denser materials.

Dye-assisted gain of strongly confined surface plasmon polaritons in silver nanowires

Noble metal nanowires are excellent candidates as subwavelength optical components in miniaturized devices due to their ability to support the propagation of surface plasmon polaritons (SPPs). Nanoscale data transfer based on SPP propagation at optical frequencies has the advantage of larger bandwidths but also suffers from larger losses due to strong mode confinement. To overcome losses, SPP gain has been realized, but so far only for weakly confined SPPs in metal films and stripes. Here we report the demonstration of gain for subwavelength SPPs that were strongly confined in chemically prepared silver nanowires (mode area = λ(2)/40) using a dye-doped polymer film as the optical gain medium. Under continuous wave excitation at 514 nm, we measured a gain coefficient of 270 cm(-1) for SPPs at 633 nm, resulting in partial SPP loss compensation of 14%. This achievement for strongly confined SPPs represents a major step forward toward the realization of nanoscale plasmonic amplifiers and lasers.

Influence of cross sectional geometry on surface plasmon polariton propagation in gold nanowires

We investigated the effects of cross sectional geometry on surface plasmon polariton propagation in gold nanowires (NWs) using bleach-imaged plasmon propagation and electromagnetic simulations. Chemically synthesized NWs have pentagonally twinned crystal structures, but recent advances in synthesis have made it possible to amplify this pentagonal shape to yield NWs with a five-pointed-star cross section and sharp end tips. We found experimentally that NWs with a five-pointed-star cross section, referred to as SNWs, had a shorter propagation length for surface plasmon polaritons at 785 nm, but a higher effective incoupling efficiency compared to smooth NWs with a pentagonal cross section, labeled as PNWs. Electromagnetic simulations revealed that the electric fields were localized at the sharp ridges of the SNWs, leading to higher absorptive losses and hence shorter propagation lengths compared to PNWs. On the other hand, scattering losses were found to be relatively uncorrelated with cross sectional geometry, but were strongly dependent on the plasmon mode excited. Our results provide insight into the shape-dependent waveguiding properties of chemically synthesized metal NWs and the mode-dependent loss mechanisms that govern surface plasmon polariton propagation.

Gold nanobelts as high confinement plasmonic waveguides

Plasmon propagation in thin plasmonic waveguides is strongly damped, making it difficult to study with diffraction-limited optics. Here we directly characterize plasmon propagation in gold nanobelts with incoherent light. The data indicate a short average propagation length of 0.94 μm but also reveal a weakly excited antisymmetric mode that has a propagation length greater than 10 μm with strong confinement of 2400 nm(2). These results demonstrate that high confinement and long propagation length can be achieved with thin plasmonic structures.

Turning the corner: efficient energy transfer in bent plasmonic nanoparticle chain waveguides

For integrating and multiplexing of subwavelength plasmonic waveguides with other optical and electric components, complex architectures such as junctions with sharp turns are necessary. However, in addition to intrinsic losses, bending losses severely limit plasmon propagation. In the current work, we demonstrate that propagation of surface plasmon polaritons around 90° turns in silver nanoparticle chains occurs without bending losses. Using a far-field fluorescence method, bleach-imaged plasmon propagation (BlIPP), which creates a permanent map of the plasmonic near-field through bleaching of a fluorophore coated on top of a plasmonic waveguide, we measured propagation lengths at 633 nm for straight and bent silver nanoparticle chains of 8.0 ± 0.5 and 7.8 ± 0.4 μm, respectively. These propagation lengths were independent of the input polarization. We furthermore show that subradiant plasmon modes yield a longer propagation length compared to energy transport via excitation of super-radiant modes.

Selective metabolic effects of gold nanorods on normal and cancer cells and their application in anticancer drug screening

Cetyltrimethylammonim bromide coated gold nanorods (Au NRs) has a potential to become anti-cancer nano-drugs. Previously, the comparative responses of human alveolar adenocarcinoma epithelial cells (A549) and normal bronchial epithelial cells (16HBE) exposed to Au NRs have been characterized. It has been shown that Au NRs are translocated from the lysosome to the mitochondria in A549 cells but not in normal 16HBE cell lines. However, the molecular information during this cellular translocation remains largely undetermined. Here, we have used a metabonomic technique to comparatively analyze the time-dependent metabolic changes in Au NRs-induced A549 and 16HBE. We found that Au NRs exposure caused a disruption in the intracellular environment of both A549 and 16HBE cells, which metabolically manifested in the reduction of lactate levels in both cell lines. In addition, Au NRs induced oxidative stress in both cells lines. However, the 16HBE cells are more able to offset the oxidative stress than the A549 cells; this is because de novo GSH synthesis is triggered in Au NRs treated 16HBE cells but not in A549 cells, and the conversion of GSH to GSSG is more profound in 16HBE cells compared to A549 cells. The severe oxidative stress induces damage to mitochondria in A549 cells, leading to cell death, which is evident in the marked reduction in the levels of nucleosides and nucleotides. Furthermore, significantly elevated levels of amino acids are likely due to stress hormones being produced in Au NRs treated cells. These findings provide comprehensive molecular information on the distinctive intracellular localization, cellular uptake and translocation of Au NRs in normal and tumor cells, highlighting the value of metabonomics in assessing biological effects of nano-drugs.

Imaging the extent of plasmon excitation in Au nanowires using pump-probe microscopy

Knowledge of how energy and charge carriers move in nanoscale systems is essential for engineering efficient devices. In this Letter, we demonstrate a technique to directly image dynamics in nanostructures based on laser scanning transient absorption microscopy, which provides near diffraction-limited spatial resolution and ultrafast time resolution. The capabilities of the technique are demonstrated by experiments on propagating surface plasmon polariton modes of Au nanowires, although these measurements can be used to study a variety of fluorescent and nonfluorescent systems.